Filmy graphite and process for producing the same

ABSTRACT

A process for producing a filmy graphite includes the steps of forming a polyimide film having a birefringence of 0.12 or more and heat-treating the polyimide film at 2,400° C. or higher.

RELATED APPLICATIONS

This application is a nationalization of PCT applicationPCT/JP2003/011221 filed on Sep. 2, 2003, the contents of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates to a filmy graphite used as aheat-dissipating film, a heat-resistant seal, a gasket, a heatingelement, or the like, and a process for producing the same.

BACKGROUND ART

Filmy graphites are important as industrial materials because of theirexcellent heat resistance, chemical resistance, high thermalconductivity, and high electrical conductivity, and are widely used asheat-dissipating materials, heat-resistant sealing materials, gaskets,heating elements, etc.

As a representative example of a process for producing an artificialfilmy graphite, a process referred to as an “expanded graphiteproduction process” is known. In this process, natural graphite isdipped in a mixed solution of concentrated sulfuric acid andconcentrated nitric acid, followed by rapid heating to produce anartificial graphite. The resulting artificial graphite is washed toremove the acids and then formed into a film with a high-pressure press.However, in the filmy graphite thus produced, strength is low and otherphysical properties are insufficient. Moreover, the residual acids alsogive rise to a problem.

In order to overcome these problems, a process has been developed inwhich a special polymer film is graphitized by direct heat treatment(hereinafter, referred to as a “polymer graphitization process”).Examples of the polymer film used for this purpose include filmscontaining polyoxadiazole, polyimide, polyphenylenevinylene,polybenzimidazole, polybenzoxazole, polythiazole, or polyamide. Thepolymer graphitization process is a process which is far simpler thanthe conventional expanded graphite production process, in which mixtureof impurities, such as acids, does not essentially occur, and which iscapable of achieving excellent thermal conductivity and electricalconductivity close to those of single crystal graphite (refer toJapanese Unexamined Patent Application Publication Nos. 60-181129,7-109171, and 61-275116).

However, the polymer graphitization process has two problems. First, itis difficult to obtain a thick filmy graphite compared with the expandedgraphite production process. Although various attempts have been made toimprove such a problem, as it now stands, transformation into a qualitygraphite is possible only when the thickness of the starting materialfilm is up to about 50 μm.

Secondly, the graphitization requires long-time heat treatment atextremely high temperatures. In general, transformation into a qualitygraphite requires heat treatment in a temperature range of 2,800° C. orhigher for at least 30 minutes.

DISCLOSURE OF INVENTION

In view of the problems associated with the conventional polymergraphitization process, it is an object of the present invention toprovide a thick filmy graphite having excellent physical properties, thefilmy graphite being produced by short-time heat treatment at relativelylow temperatures.

In order to overcome the problems described above, the present inventorshave taken notice of a polyimide which represents a graphitizablepolymer, and it has been attempted to graphitize various polyimidefilms. As a result, it has been found that by controlling the molecularstructure and molecular orientation of the polyimide, transformationinto a quality graphite is enabled. More specifically, it has been foundthat the birefringence or coefficient of linear expansion, which is aphysical property of a polyimide film, can be the most direct indicatorof whether the polyimide film can be transformed into a qualitygraphite. Here, the coefficient of linear expansion is defined as acoefficient of linear expansion in a direction parallel to the filmplane.

That is, according to the present invention, a process for producing afilmy graphite includes the steps of forming a polyimide film having abirefringence of 0.12 or more and heat-treating the polyimide film at2,400° C. or higher.

Alternatively, a process for producing a filmy graphite may include thesteps of forming a polyimide film having a mean coefficient of linearexpansion of less than 2.5×10⁻⁵/° C. in a range of 100° C. to 200° C.,the mean coefficient of linear expansion being in a planar direction ofthe film, and heat-treating the polyimide film at 2,400° C. or higher.

Preferably, a process for producing a filmy graphite includes the stepsof forming a polyimide film having a mean coefficient of linearexpansion of less than 2.5×10⁻⁵/° C. in a range of 100° C. to 200° C.,the mean coefficient of linear expansion being in a planar direction ofthe film, and having a birefringence of 0.12 or more, and heat-treatingthe polyimide film at 2,400° C. or higher.

In the process for producing the filmy graphite, the polyimide film maybe formed using, as a starting material, an acid dianhydride representedby chemical formula 1:

wherein R₁ represents any one of divalent organic groups represented bychemical formulae 2:

wherein R₂, R₃, R₄, and R₅ each represent any one selected from thegroup consisting of —CH₃, —Cl, —Br, —F, and —OCH₃.

In the process for producing the filmy graphite, preferably, thepolyimide film is formed using, as a starting material, an aciddianhydride represented by chemical formula 3:

In the process for producing the filmy graphite, it is also preferableto form the polyimide film using pyromellitic dianhydride orp-phenylenediamine as a starting material. In the process for producingthe filmy graphite, the polyimide film may be formed by treating apolyamic acid, which is a precursor, with a dehydrating agent and animidization accelerator. Preferably, the polyimide film is formed bysynthesizing a pre-polymer using a first diamine and an aciddianhydride, the pre-polymer having the acid dianhydride moiety at bothtermini, synthesizing a polyamic acid by allowing the pre-polymer toreact with a second diamine, and imidizing the polyamic acid.

An artificial filmy graphite according to the present invention can havea thickness of 30 μm or more and a thermal diffusivity of 8.5×10⁻⁴ m²/sor more. Preferably, the artificial filmy graphite can have a thicknessof 3 μm or more and a thermal diffusivity of 10×10⁻⁴ m²/s or more.

The artificial filmy graphite can have a thickness of 30 μm or more andan electrical conductivity of 8.5×10⁴ S·cm or more. In the filmygraphite, the ratio of electrical resistance at 77 K to that at roomtemperature can be 1.5 or less, and the ratio of electrical resistanceat 4 K to that at room temperature can be 1.4 or less.

The artificial filmy graphite can have a thickness of 30 μm or more anda density of 2.15 g/mm³ or more. In the artificial filmy graphite havinga thickness of 30 μm or more, when a light beam with a diameter of 10 μmis applied to the center of a cross section in the thickness direction,with respect to Raman-scattered light, the ratio of peak height at awave number of 1,310 cm⁻¹ to that at a wave number of 1,580 cm⁻¹ can be0.35 or less.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plan view illustrating cutting-out of a specimen formeasuring birefringence of a polyimide film.

FIG. 2 is a perspective view of the specimen for measuring birefringencecut out as illustrated in FIG. 1.

FIG. 3 is a bright-field image of a filmy graphite in the vicinity of asurface layer in an example of the present invention, the image beingobserved with a transmission electron microscope (TEM).

FIG. 4 is a lattice image of a filmy graphite in the vicinity of asurface layer in an example of the present invention, the image beingobserved with a TEM.

FIG. 5 is a bright-field image of a filmy graphite in the vicinity of acenter in the thickness direction in an example of the presentinvention, the image being observed with a TEM.

FIG. 6 is a lattice image of a filmy graphite in the vicinity of acenter in the thickness direction in an example of the presentinvention, the image being observed with a TEM.

FIG. 7 is a bright-field image of a filmy graphite in the vicinity of asurface layer in a comparative example, the image being observed with aTEM.

FIG. 8 is a lattice image of a filmy graphite in the vicinity of asurface layer in a comparative example, the image being observed with aTEM.

FIG. 9 is a bright-field image of a filmy graphite in the vicinity of acenter in the thickness direction in a comparative example, the imagebeing observed with a TEM.

FIG. 10 is a lattice image of a filmy graphite in the vicinity of acenter in the thickness direction in a comparative example, the imagebeing observed with a TEM.

BEST MODE FOR CARRYING OUT THE INVENTION

In a polyimide film used in the present invention, the birefringence Δn,which is associated with in-plane orientation of molecules, is 0.12 ormore, preferably 0.14 or more, and most preferably 0.16 or more, in anyin-plane direction of the film. The birefringence of the film lower than0.12 indicates poorer in-plane orientation of molecules of the film.Graphitization of such a film requires heating to a higher temperatureand a longer heat-treating time. Furthermore, the resulting filmygraphite tends to have inferior electrical conductivity, thermalconductivity, and mechanical strength.

On the other hand, at a birefringence of 0.12 or more, in particular,0.14 or more, the maximum temperature can be lowered and theheat-treating time can be shortened. Furthermore, since the resultingfilmy graphite has improved crystal orientation, the electricalconductivity, thermal conductivity, and mechanical strength thereof areremarkably improved. Although the reason for this is not clear, it isassumed that rearrangement of molecules is required for graphitization,and in the polyimide having excellent molecular orientation, therequired rearrangement of molecules is minimal, thus enablinggraphitization at relatively low temperatures.

Herein, the term “birefringence” means a difference between a refractiveindex in any in-plane direction of a film and a refractive index in thethickness direction. The birefringence Δnx in an in-plane direction X isgiven by the following expression:Birefringence Δnx=(refractive index Nx in in-planedirection X)−(refractive index Nz in thickness direction)

FIGS. 1 and 2 illustrate a specific method for measuring birefringence.Referring to a plan view of FIG. 1, a wedge-shaped sheet 2 is cut out asa measurement specimen from a film 1. The wedge-shaped sheet 2 has along trapezoidal shape with an oblique line, and one base angle thereofis a right angle. The wedge-shaped sheet 2 is cut out such that thebottom of the trapezoid is parallel to the X direction. FIG. 2 is aperspective view of the measurement specimen 2 thus cut out. Sodiumlight 4 is applied at right angles to a cutout cross-sectioncorresponding to the bottom of the trapezoidal specimen 2, and a cutoutcross-section corresponding to the oblique line of the trapezoidalspecimen 2 is observed with a polarization microscope. Thereby,interference fringes 5 are observed. The birefringence Δnx in thein-plane direction X is represented by the expression:Δnx=n×λ/dwhere n is the number of interference fringes, λ is the wavelength ofsodium D ray, i.e., 589 nm, and d is the width 3 of the specimencorresponding to the height of the trapezoid of the specimen 2.

Note that the term “in an in-plane direction X of a film” means that,for example, the X direction is any one of in-plane directions of 0degrees, 45 degrees, 90 degrees, and 135 degrees on the basis of thedirection of flow of materials during the formation of the film.

Furthermore, the polyimide film used in the present invention, which isa starting material for the filmy graphite, has a mean coefficient oflinear expansion of less than 2.5×10⁻⁵/° C. in a range of 100° C. to200° C. By using such a polyimide film as a starting material,transformation into a graphite starts from 2,400° C. and transformationinto a graphite of sufficiently good quality can take place at 2,700° C.Moreover, in comparison with a case in which a polyimide film having acoefficient of linear expansion of 2.5×10⁻⁵/° C. or more conventionallyknown as a starting material for a filmy graphite is used, in thepolyimide film having a coefficient of linear expansion of less than2.5×10⁻⁵/° C., transformation into a graphite is enabled at lowertemperatures even at the same thickness. That is, even if a film that isthicker than the conventional film is used as a starting material,graphitization is allowed to proceed easily. More preferably, thecoefficient of linear expansion is 2.0×10⁻⁵/° C. or less.

If the coefficient of linear expansion of the film is 2.5×10⁻⁵/° C. ormore, the change during heat treatment increases, graphitization becomesdisordered, and brittleness occurs. The resulting filmy graphite tendsto have low electrical conductivity, thermal conductivity, andmechanical strength. On the other hand, if the coefficient of linearexpansion is less than 2.5×10⁻⁵/° C., elongation during heat treatmentis small, graphitization proceeds smoothly, and brittleness does notoccur. As a result, it is possible to obtain a filmy graphite that isexcellent in various properties.

Note that the coefficient of linear expansion of the film is obtained bythe following method. Using a thermomechanical analyzer (TMA), aspecimen is heated to 350° C. at a heating rate of 10° C./min and thenair-cooled to room temperature. The specimen is heated again to 350° C.at a heating rate of 10° C./min, and the mean coefficient of linearexpansion at 100° C. to 200° C. during the second heating is measured.Specifically, using a thermomechanical analyzer (TMA: SSC/5200H; TMA120Cmanufactured by Seiko Electronics Industry Co., Ltd.), a film specimenwith dimensions of 3 mm in width and 20 mm in length is fixed on apredetermined jig, and measurement is performed in the tensile modeunder a load of 3 g in a nitrogen atmosphere.

Furthermore, the polyimide film used in the present invention preferablyhas an elastic modulus of 350 kgf/mm² or more from the standpoint thatgraphitization can be more easily performed. That is, if the elasticmodulus is 350 kgf/mm² or more, heat treatment can be performed whileapplying a tension to the polyimide film, and it is possible to avoidbreakage of the film resulting from shrinkage of the film during heattreatment. Thus, it is possible to obtain a filmy graphite that isexcellent in various properties.

Note that the elastic modulus of the film can be measured in accordancewith ASTM-D-882. The polyimide film more preferably has an elasticmodulus of 400 kgf/mm² or more, and still more preferably 500 kgf/mm² ormore. If the elastic modulus of the film is less than 350 kgf/mm²,breakage and deformation easily occur due to shrinkage of the filmduring heat treatment, and the resulting filmy graphite tends to havelow electrical conductivity, thermal conductivity, and mechanicalstrength.

The polyimide film used in the present invention can be formed byflow-casting an organic solution of a polyamic acid which is a precursorof the polyimide onto a support, such as an endless belt or stainlesssteel drum, followed by drying and imidization.

A known process can be used as the process for producing the polyamicacid used in the present invention. Usually, at least one aromatic aciddianhydride and at least one diamine are dissolved in substantiallyequimolar amounts in an organic solvent. The resulting organic solutionis stirred under controlled temperature conditions until polymerizationbetween the acid dianhydride and the diamine is completed. Thereby, apolyamic acid is produced. Such a polyamic acid solution is obtainedusually at a concentration of 5% to 35% by weight, and preferably 10% to30% by weight. When the concentration is in such a range, a propermolecular weight and solution viscosity can be obtained.

As the polymerization method, any of the known methods can be used. Forexample, the following polymerization methods (1) to (5) are preferable.

(1) A method in which an aromatic diamine is dissolved in a polarorganic solvent, and a substantially equimolar amount of an aromatictetracarboxylic dianhydride is allowed to react therewith to performpolymerization.

(2) A method in which an aromatic tetracarboxylic dianhydride and a lessthan equimolar amount of an aromatic diamine compound with respectthereto are allowed to react with each other in a polar organic solventto obtain a pre-polymer having acid anhydride groups at both termini.Subsequently, polymerization is performed using an aromatic diaminecompound so as to be substantially equimolar with respect to thearomatic tetracarboxylic dianhydride.

(3) A method in which an aromatic tetracarboxylic dianhydride and anexcess molar amount of an aromatic diamine compound with respect theretoare allowed to react with each other in a polar organic solvent toobtain a pre-polymer having amino groups at both termini. Subsequently,after adding an additional aromatic diamine compound to the pre-polymer,polymerization is performed using an aromatic tetracarboxylicdianhydride such that the aromatic tetracarboxylic dianhydride and thearomatic diamine compound are substantially equimolar to each other.

(4) A method in which an aromatic tetracarboxylic dianhydride isdissolved and/or dispersed in a polar organic solvent, and thenpolymerization is performed using an aromatic diamine compound so as tobe substantially equimolar to the acid dianhydride.

(5) A method in which a mixture of substantially equimolar amounts of anaromatic tetracarboxylic dianhydride and an aromatic diamine are allowedto react with each other in a polar organic solvent to performpolymerization.

Among these, as in methods (2) and (3), a method in which sequentialcontrol is used by way of a pre-polymer to perform polymerization ispreferable. The reason for this is that by using sequential control, itis possible to easily obtain a polyimide film having a low birefringenceand a low coefficient of linear expansion. By heat-treating thispolyimide film, it becomes possible to easily obtain a filmy graphitehaving excellent electrical conductivity, thermal conductivity, andmechanical strength. Furthermore, it is assumed that since thepolymerization reaction is regularly controlled, the overlap betweenaromatic rings increases, and graphitization is allowed to proceedeasily even by low-temperature heat treatment.

In the present invention, examples of the acid dianhydride which can beused for the synthesis of the polyimide include pyromelliticdianhydride, 2,3,6,7-naphthalenetetracarboxylic dianhydride,3,3′,4,4′-biphenyltetracarboxylic dianhydride,1,2,5,6-naphthalenetetracarboxylic dianhydride,2,2′,3,3′-biphenyltetracarboxylic dianhydride,3,3′,4,4′-benzophenonetetracarboxylic dianhydride,2,2-bis(3,4-dicarboxyphenyl)propane dianhydride,3,4,9,10-perylenetetracarboxylic dianhydride,bis(3,4-dicarboxyphenyl)propane dianhydride,1,1-bis(2,3-dicarboxyphenyl)ethane dianhydride,1,1-bis(3,4-dicarboxyphenyl)ethane dianhydride,bis(2,3-dicarboxyphenyl)methane dianhydride,bis(3,4-dicarboxyphenyl)ethane dianhydride, oxydiphthalic dianhydride,bis(3,4-dicarboxyphenyl)sulfone dianhydride, p-phenylenebis(trimelliticacid monoester anhydride), ethylenebis(trimellitic acid monoesteranhydride), bisphenol A bis(trimellitic acid monoester anhydride), andanalogues thereof. These may be used alone or in appropriate combinationof two or more.

In the present invention, examples of the diamine which can be used forthe synthesis of the polyimide include 4,4′-oxydianiline,p-phenylenediamine, 4,4′-diaminodiphenylpropane,4,4′-diaminodiphenylmethane, benzidine, 3,3′-dichlorobenzidine,4,4′-diaminodiphenyl sulfide, 3,3′-diaminodiphenyl sulfone,4,4′-diaminodiphenyl sulfone, 4,4′-diaminodiphenyl ether,3,3′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether,1,5-diaminonaphthalene, 4,4′-diaminodiphenyldiethylsilane,4,4′-diaminodiphenylsilane, 4,4′-diaminodiphenylethylphosphine oxide,4,4′-diaminodiphenyl-N-methylamine, 4,4′-diaminodiphenyl-N-phenylamine,1,4-diaminobenzene(p-phenylenediamine), 1,3-diaminobenzene,1,2-diaminobenzene, and analogues thereof. These may be used alone or inappropriate combination of two or more.

In particular, from the standpoint that the coefficient of linearexpansion can be decreased, the elastic modulus can be increased, andthe birefringence can be increased, use of an acid dianhydriderepresented by chemical formula 1 below as a starting material ispreferable in the production of the polyimide film in the presentinvention.

In the formula, R₁ represents any one of divalent organic groupsrepresented by chemical formulae 2:

wherein R₂, R₃, R₄, and R₅ each represent any one selected from thegroup consisting of —CH₃, —Cl, —Br, —F, and —CH₃O.

By using the acid dianhydride described above, it is possible to obtaina polyimide film having a relatively low coefficient of waterabsorption, which is also preferable from the standpoint that foamingdue to moisture can be prevented in the graphitization process.

In particular, use of any one of the benzene nucleus-containing organicgroups represented by chemical formulae 2 as R₁ in the acid dianhydrideis preferable from the standpoint that the resulting polyimide film hashigh molecular orientation, a low coefficient of linear expansion, ahigh elastic modulus, a high birefringence, and a low coefficient ofwater absorption.

An acid dianhydride represented by molecular formula 3 below may be usedas a starting material in the synthesis of the polyimide in the presentinvention to further decrease the coefficient of linear expansion,increase the elastic modulus, increase the birefringence, and decreasethe coefficient of water absorption.

In particular, with respect to a polyimide film produced using, as astarting material, an acid dianhydride having a structure in whichbenzene rings are linearly bonded by two or more ester bonds, althoughfolded chains are involved, a highly linear conformation is easilyformed as a whole, and the polyimide film has a relatively rigidproperty. As a result, by using this starting material, it is possibleto decrease the coefficient of linear expansion of the polyimide film,for example, to 1.5×10⁻⁵/° C. or less. In addition, the elastic moduluscan be increased to 500 kgf/mm² or more, and the coefficient of waterabsorption can be decreased to 1.5% or less.

The polyimide of the present invention is preferably synthesized usingp-phenylenediamine as a starting material to further decrease thecoefficient of linear expansion, increase the elastic modulus, andincrease the birefringence.

In the present invention, the acid dianhydride most suitably used forthe synthesis of the polyimide film includes pyromellitic dianhydrideand/or p-phenylenebis(trimellitic acid monoester dianhydride)represented by (Chemical Formula 3). The number of moles of one of theseor both is preferably 40 mole percent or more, more preferably 50 molepercent or more, even more preferably 70 mole percent or more, and stillmore preferably 80 mole percent or more relative to the total aciddianhydride content. If the amount of use of these acid dianhydrides isless than 40 mole percent, the resulting polyimide film tends to have anincreased coefficient of linear expansion, a decreased elastic modulus,and a decreased birefringence.

Furthermore, in the present invention, the diamine most suitably usedfor the synthesis of the polyimide includes 4,4′-oxydianiline andp-phenylenediamine. The number of moles of one of these or both ispreferably 40 mole percent or more, more preferably 50 mole percent ormore, even more preferably 70 mole percent or more, and still morepreferably 80 mole percent or more relative to the total diaminecontent. Furthermore, p-phenylenediamine is included preferably in anamount of 10 mole percent or more, more preferably 20 mole percent ormore, even more preferably 30 mole percent or more, and still morepreferably 40 mole percent or more. If the contents of these diaminesare below the lower limits of these mole percent ranges, the resultingpolyimide film tends to have an increased coefficient of linearexpansion, a decreased elastic modulus, and a decreased birefringence.However, if the total diamine content is entirely composed ofp-phenylenediamine, it is difficult to obtain a thick polyimide filmwhich does not substantially foam. Therefore, use of 4,4′-oxydianilineis preferable.

Preferred examples of the solvent for the synthesis of the polyamic acidinclude amide solvents, such as N,N-dimethylformamide,N,N-dimethylacetamide, and N-methyl-2-pyrrolidone, andN,N-dimethylformamide and N,N-dimethylacetamide are particularlypreferably used.

The polyimide may be produced using either a thermal cure method or achemical cure method. In the thermal cure method, a polyamic acid, whichis a precursor, is imidized by heating. In the chemical cure method, apolyamic acid is imidized using a dehydrating agent represented by anacid anhydride, such as acetic anhydride, and a tertiary amine, such aspicoline, quinoline, isoquinoline, or pyridine, as an imidizationaccelerator. Above all, a tertiary amine having a higher boiling point,such as isoquinoline, is more preferable. The reason for this is thatsuch a tertiary amine is not evaporated in the initial stage of theproduction process of the film and tends to exhibit a catalytic effectuntil the final step of drying.

In particular, from the standpoints that the resulting film tends tohave a low coefficient of linear expansion, a high elastic modulus, anda high birefringence and that rapid graphitization is enabled atrelatively low temperatures and a quality graphite can be obtained,chemical curing is preferable. Furthermore, combined use of thedehydrating agent and the imidization accelerator is preferable becausethe resulting film can have a decreased coefficient of linear expansion,an increased elastic modulus, and an increased birefringence. Moreover,in the chemical cure method, since imidization reaction proceeds morerapidly, the imidization reaction can be completed for a short period oftime in heat treatment. Thus, the chemical cure method has highproductivity and is industrially advantageous.

In a specific process for producing a film using chemical curing, first,stoichiometric amounts or more of a dehydrating agent and an imidizationaccelerator composed of a catalyst are added to a polyamic acidsolution, the solution is flow-cast or applied onto a support, e.g., asupporting plate, an organic film, such as PET, a drum, or an endlessbelt, so as to be formed into a film, and an organic solvent isevaporated to obtain a self-supporting film. Subsequently, theself-supporting film is imidized while drying by heating to obtain apolyimide film. The heating temperature is preferably in a range of 150°C. to 550° C.

Although the heating rate is not particularly limited, preferably,gradual heating is performed continuously or stepwise so that thehighest temperature reaches the predetermined temperature range. Theheating time depends on the thickness of the film and the highesttemperature. In general, the heating time is preferably 10 seconds to 10minutes after the highest temperature is achieved. Moreover, it ispreferable to include a step of fixing and drawing the film in order toprevent shrinkage in the production process of the polyimide filmbecause the resulting film tends to have a small coefficient of linearexpansion, a high elastic modulus, and a high birefringence.

In the graphitization process of the polyimide film, in the presentinvention, the polyimide film, which is a starting material, issubjected to preheat treatment under reduced pressure or in nitrogen gasto perform carbonization. The preheating is usually carried out at about1,000° C., and for example, when the temperature is raised at a rate of10° C./min, preferably, the film is retained for about 30 minutes in atemperature range of about 1,000° C. In the stage of temperature rise,in order to prevent loss of molecular orientation of the startingpolymer film, preferably, a pressure is applied in a directionperpendicular to the surface of the film to an extent that does notcause breakage of the film.

Subsequently, the carbonized film is set in a very high temperature ovento perform graphitization. The graphitization is performed in an inertgas. As the inert gas, argon is suitable, and addition of a small amountof helium to argon is more preferable. The heat treatment temperaturerequired is at least 2,400° C. at the minimum, and heat treatment isfinally performed preferably at a temperature of 2,700° C. or higher,and more preferably 2,800° C. or higher.

As the heat treatment temperature is increased, transformation into aquality graphite is more easily enabled. However, in view of economics,preferably, transformation into a quality graphite is enabled attemperatures as low as possible. In order to achieve a very hightemperature of 2,500° C. or higher, usually, a current is directlyapplied to a graphite heater and heating is performed using theresulting Joule heat. Deterioration of the graphite heater advances at2,700° C. or higher. At 2,800° C., the deterioration rate increasesabout tenfold, and at 2,900° C., the deterioration rate increasesfurther about tenfold. Consequently, it brings about a large economicaladvantage to decrease the temperature at which transformation into aquality graphite is enabled, for example, from 2,800° C. to 2,700° C.,by improving the polymer film as the starting material. Note that in agenerally available industrial oven, the maximum temperature at whichheat treatment can be performed is limited to 3,000° C.

In the graphitization treatment, the carbonized film produced by thepreheat treatment is transformed so as to have a graphite structure.During this treatment, cleavage and recombination of carbon-carbon bondsmust occur. In order to cause graphitization at temperatures as low aspossible, it is necessary to allow the cleavage and recombination tooccur at minimum energy. The molecular orientation of the startingpolyimide film affects the arrangement of carbon atoms in the carbonizedfilm, and the molecular orientation can produce an effect of decreasingthe energy of cleavage and recombination of carbon-carbon bonds duringgraphitization. Consequently, by designing molecules so that highmolecular orientation easily occurs, graphitization at relatively lowtemperatures is enabled. By using two-dimensional molecular orientationparallel to the surface of the film, the effect of the molecularorientation becomes more remarkable.

The second characteristic of the graphitization reaction is thatgraphitization does not easily proceed at low temperatures if thecarbonized film is thick. Consequently, when a thick carbonized film isgraphitized, a state may occur in which the graphite structure is formedin a surface layer while the graphite structure is not formed yet in aninterior region. The molecular orientation of the carbonized filmpromotes graphitization in the interior region of the film, and as aresult, transformation into a quality graphite is enabled at lowertemperatures.

Substantially simultaneous progress of graphitization in the surfacelayer and in the interior region of the carbonized film is also usefulin avoiding the situation in which the graphite structure formed in thesurface layer is destroyed by a gas generated from inside, andgraphitization of a thicker film is enabled. The polyimide film formedin the present invention is believed to have molecular orientation thatis most suitable for producing such an effect.

As described above, by using the polyimide film formed in the presentinvention, it becomes possible to graphitize a film that is thicker thanconventional graphitizable polyimide films. Specifically, even in a filmwith a thickness of 200 μm, transformation into a quality filmy graphiteis enabled by selecting an appropriate heat treatment.

Various examples of the present invention together with severalcomparative examples will be described below.

EXAMPLE 1

Pyromellitic dianhydride (4 equivalents) was dissolved in a solutionprepared by dissolving 3 equivalents of 4,4′-oxydianiline and 1equivalent of p-phenylenediamine in dimethylformamide (DMF) to produce asolution containing 18.5% by weight of polyamic acid.

While cooling the resulting solution, an imidization catalyst containing1 equivalent of acetic anhydride and 1 equivalent of isoquinoline,relative to the carboxylic acid group contained in the polyamic acid,and DMF was added thereto, followed by defoaming. Subsequently, theresulting mixed solution was applied onto an aluminum foil such that apredetermined thickness was achieved after drying. The mixed solutionlayer on the aluminum foil was dried using a hot-air oven and afar-infrared heater.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the aluminum foil was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the aluminum foil andfixed on a frame. The gel film was dried by heating stepwise in ahot-air oven at 120° C. for 30 seconds, at 275° C. for 40 seconds, at400° C. for 43 seconds, and at 450° C. for 50 seconds, and with afar-infrared heater at 460° C. for 23 seconds. With respect to otherthicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm.

Five types of polyimide films with thicknesses of 25 μm, 50 μn, 75 μm,100 μm, and 200 μm (Sample A: elastic modulus 400 kgf/mm², coefficientof water absorption>2.0%) were produced.

Sample A was sandwiched between graphite plates, and using a very hightemperature oven provided with a graphite heater, preliminary treatmentwas performed in which the temperature was raised to 1,000° C. at a rateof 16.7° C./min under reduced pressure. Subsequently, using a very hightemperature oven, under a pressurized argon atmosphere of 0.8 kgf/cm²,the temperature was raised to 2,700° C. at a rate of 7° C./min.Furthermore, under a pressurized argon atmosphere of 0.8 kgf/cm², thetemperature was raised to 2,800° C., the maximum temperature, at a rateof 2° C./min, and Sample A was retained for one hour at the maximumtemperature. Cooling was then performed to obtain filmy graphites.

The progress of graphitization was determined by measuring electricalconductivity and thermal diffusivity in a planar direction of the film.That is, higher electrical conductivity and higher thermal diffusivityindicate increased graphitization. The results thereof are shown inTable 1. In the case of the polyimide (Sample A) in Example 1, the heattreatment at 2,700° C. already causes transformation into qualitygraphites, and excellent electrical conductivity and thermalconductivity are exhibited. As is evident from the results, by using thepolyimide of Example 1, it is possible to graphitize a polyimide filmthat is thicker than a polyimide film of conventional Kapton (registeredtrademark) type shown in Comparative Example 1 which will be describedbelow, and transformation into a quality graphite is enabled even at atemperature of 2,700° C., which is 100° C. lower than the commongraphitization temperature of the Kapton type polyimide film, i.e.,2,800° C.

The electrical conductivity was measured by the four-terminal method.Specifically, a filmy graphite sample with a size of about 3 mm×6 mm wasprepared. After confirming that no breaks or wrinkles were present withan optical microscope, a pair of outer electrodes were attached to bothends of the sample using silver paste, and a pair of inner electrodeswere attached inside between the outer electrodes using silver paste.Using a constant current source (“Programmable Current Source 220”available from Keithley Instruments, Inc.), a constant current of 1 mAwas applied between the outer electrodes, and the voltage between theinner electrodes was measured with a voltmeter (“Nanovoltmeter”available from Keithley Instruments, Inc.). The electrical conductivitywas calculated according to the expression: (applied current/measuredvoltage)×(distance between inner electrodes/cross-sectional area ofsample).

The thermal diffusivity was measured with a thermal diffusivity meterusing an AC method (“LaserPit” available from ULVAC-RIKO, Inc.), underan atmosphere of 20° C., at 10 Hz.

TABLE 1 Starting film Heat treatment Coefficient of temperatureThickness linear expansion Electrical conductivity Thermal diffusivity(° C.) (μm) (×10⁻⁵/° C.) Birefringence (S · cm) (10⁻⁴ m²/s) 2700 25 1.80.14 11,500 8.4 50 1.8 0.14 11,000 8.3 75 1.9 0.14 10,000 8.1 100 1.90.14 9,700 8.0 200 2.0 0.14 9,800 8.0 2800 25 1.8 0.14 12,000 8.7 50 1.80.14 11,000 8.5 75 1.9 0.14 11,000 8.5 100 1.9 0.14 10,500 8.5 200 2.00.14 10,000 8.5

COMPARATIVE EXAMPLE 1

Pyromellitic dianhydride (1 equivalent) was dissolved in a solutionprepared by dissolving 1 equivalent of 4,4′-oxydianiline in DMF toproduce a solution containing 18.5% by weight of polyamic acid.

While cooling the resulting the resulting solution, an imidizationcatalyst containing 1 equivalent of acetic anhydride and 1 equivalent ofisoquinoline, relative to the carboxylic acid group contained in thepolyamic acid, and DMF was added thereto, followed by defoaming.Subsequently, the resulting mixed solution was applied onto an aluminumfoil such that a predetermined thickness was achieved after drying. Themixed solution layer on the aluminum foil was dried using a hot-air ovenand a far-infrared heater.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the aluminum foil was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the aluminum foil andfixed on a frame. The gel film was dried by heating stepwise in ahot-air oven at 120° C. for 30 seconds, at 275° C. for 40 seconds, at400° C. for 43 seconds, and at 450° C. for 50 seconds, and with afar-infrared heater at 460° C. for 23 seconds. With respect to otherthicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm.

Five types of conventional polyimide films of typical Kapton (registeredtrademark) type with thicknesses of 25 μm, 50 μm, 75 μm, 100 μm, and 200μm (elastic modulus 300 kgf/mm², coefficient of water absorption>2.0%)were produced. Using these films, graphitization was performed also inthis comparative example by the same method as that in Example 1.

The properties of the filmy graphites produced in Comparative Example 1are shown in Table 2. As is evident from Table 2, when the films with athickness of 75 μm or more are used, the resulting graphites have poorelectrical conductivity and thermal diffusivity. Only in the polyimidefilms with thicknesses of 25 μm and 50 μm, high graphitization isachieved. As is also evident from Table 2, the properties of thegraphitized films obtained by the heat treatment at 2,700° C. areconsiderably inferior to those of the case in which the polyimide(Sample A) of Example 1 is used.

From the comparison between Comparative Example 1 and Example 1described above, the superiority of the polyimide of the presentinvention in the graphitization reaction is apparent.

TABLE 2 Starting film Heat treatment Coefficient of temperatureThickness linear expansion Electrical conductivity Thermal diffusivity(° C.) (μm) (×10⁻⁵/° C.) Birefringence (S · cm) (10⁻⁴ m²/s) 2700 25 3.20.11 9,500 7.2 50 3.1 0.10 9,000 6.3 75 3.2 0.10 5,000 4.0 100 3.1 0.101,200 2.0 200 3.1 0.10 800 1.5 2800 25 3.2 0.11 11,500 8.0 50 3.1 0.1010,000 7.8 75 3.2 0.10 7,000 4.5 100 3.1 0.10 4,500 3.0 200 3.1 0.101,000 1.8

EXAMPLE 2

Pyromellitic dianhydride (3 equivalents) was dissolved in a solutionprepared by dissolving 2 equivalents of 4,4′-oxydianiline and 1equivalent of p-phenylenediamine in dimethylformamide (DMF) to produce asolution containing 15% by weight of polyamic acid.

While cooling the resulting solution, an imidization catalyst containing1 equivalent of acetic anhydride and 1 equivalent of isoquinoline,relative to the carboxylic acid group contained in the polyamic acid,and DMF was added thereto, followed by defoaming. Subsequently, theresulting mixed solution was applied onto an aluminum foil such that apredetermined thickness was achieved after drying. The mixed solutionlayer on the aluminum foil was dried using a hot-air oven and afar-infrared heater.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the aluminum foil was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the aluminum foil andfixed on a frame. The gel film was dried by heating stepwise in ahot-air oven at 120° C. for 30 seconds, at 275° C. for 40 seconds, at400° C. for 43 seconds, and at 450° C. for 50 seconds, and with afar-infrared heater at 460° C. for 23 seconds. With respect to otherthicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm.

When compared with Example 1, the percentage of p-phenylenediamine,i.e., a rigid component, was high in Example 2, and the resultingpolyimide films had higher molecular orientation. Consequently, in thecase of thick films, the solvent and the catalyst were caught in theresin, and foaming easily occurred because of the evaporation of thesolvent of the polyimide film and the imidization catalyst. In order toprevent foaming, it was necessary to set the firing time at lowtemperatures to be sufficiently long.

Four types of polyimide films with thicknesses of 25 μm, 50 μm, 75 μm,and 100 μm (Sample B: elastic modulus 450 kg/mm², coefficient of waterabsorption>2.0%) were produced. Using these films, graphitization wasperformed also in Example 2 by the same method as that in Example 1.

The properties of the filmy graphites produced in Example 2 are shown inTable 3. As is evident from comparison between Tables 3 and 1, theproperties of the filmy graphites obtained in Example 2 are slightlysuperior to those in Example 1.

TABLE 3 Starting film Heat treatment Coefficient of temperatureThickness linear expansion Electrical conductivity Thermal diffusivity(° C.) (μm) (×10⁻⁵/° C.) Birefringence (S · cm) (10⁻⁴ m²/s) 2700 25 1.20.15 11,500 8.5 50 1.1 0.16 11,000 8.3 75 1.2 0.16 10,000 8.2 100 1.10.15 9,700 8.1 2800 25 1.2 0.15 12,500 8.9 50 1.1 0.16 11,000 8.8 75 1.20.16 10,500 8.7 100 1.1 0.15 10,500 8.7

EXAMPLE 3

Pyromellitic dianhydride (1 equivalent) and p-phenylenebis(trimelliticacid monoester anhydride) were dissolved in a solution prepared bydissolving 1 equivalent of 4,4′-oxydianiline and 1 equivalent ofp-phenylenediamine in DMF to produce a solution containing 15% by weightof polyamic acid.

While cooling the resulting solution, an imidization catalyst containing1 equivalent of acetic anhydride and 1 equivalent of isoquinoline,relative to the carboxylic acid group contained in the polyamic acid,and DMF was added thereto, followed by defoaming. Subsequently, theresulting mixed solution was applied onto an aluminum foil such that apredetermined thickness was achieved after drying. The mixed solutionlayer on the aluminum foil was dried using a hot-air oven and afar-infrared heater.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the aluminum foil was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the aluminum foil andfixed on a frame. The gel film was dried by heating stepwise in ahot-air oven at 120° C. for 30 seconds, at 275° C. for 40 seconds, at400° C. for 43 seconds, and at 450° C. for 50 seconds, and with afar-infrared heater at 460° C. for 23 seconds. With respect to otherthicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm.

When compared with Example 1, the resulting polyimide films had highermolecular orientation in Example 3. Consequently, in the case of thickfilms, the solvent and the catalyst were caught in the resin, andfoaming easily occurred because of evaporation of the solvent of thepolyimide film and the imidization catalyst. In order to preventfoaming, it was necessary to set the firing time at low temperatures tobe sufficiently long.

Four types of polyimide films with thicknesses of 25 μm, 50 μm, 75 μm,and 100 μm (Sample C: elastic modulus 500 kg/mm², coefficient of waterabsorption>1.5%) were produce. Using these films, graphitization wasperformed also in Example 3 by the same method as that in Example 1.

The properties of the filmy graphites produced in Example 3 are shown inTable 4. As is evident from comparison between Tables 4 and 1, theproperties of the filmy graphites obtained in Example 3 weresubstantially the same as those in Example 1.

TABLE 4 Starting film Heat treatment Coefficient of temperatureThickness linear expansion Electrical conductivity Thermal diffusivity(° C.) (μm) (×10⁻⁵/° C.) Birefringence (S · cm) (10⁻⁴ m²/s) 2700 25 0.90.16 10,000 8.3 50 1.0 0.16 10,000 8.3 75 0.9 0.15 9,500 8.1 100 1.00.15 9,300 8.1 2800 25 0.9 0.16 11,300 8.5 50 1.0 0.16 11,000 8.3 75 0.90.15 10,000 8.2 100 1.0 0.15 9,500 8.2

EXAMPLE 4

Pyromellitic dianhydride (4 equivalents) was dissolved in a solutionprepared by dissolving 3 equivalents of 4,4′-oxydianiline and 1equivalent of p-phenylenediamine in dimethylformamide (DMF) to produce asolution containing 18.5% by weight of polyamic acid.

While cooling the resulting solution, an imidization catalyst containing1 equivalent of isoquinoline relative to the carboxylic acid groupcontained in the polyamic acid and DMF was added thereto, followed bydefoaming. Subsequently, the resulting mixed solution was applied ontoan aluminum foil such that a predetermined thickness was achieved afterdrying. The mixed solution layer on the aluminum foil was dried using ahot-air oven.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the aluminum foil was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the aluminum foil andfixed on a frame. The gel film was dried by heating stepwise in ahot-air oven at 120° C. for 30 minutes, at 275° C. for 30 minutes, at400° C. for 30 minutes, and at 450° C. for 30 minutes. With respect toother thicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm. Furthermore, when the thickness of the film was large, in order toprevent foaming because of the evaporation of the solvent of thepolyimide film and the imidization catalyst, the firing time at lowtemperatures was set to be sufficiently long. In particular, when aceticanhydride is not added as in Example 4, the reaction proceeds slowly andthe polarity does not change. As a result, the exudation of the solventand the catalyst slows down, and foaming easily occurs during theformation of the polyimide film. Consequently, when a thick polyimidefilm is formed, adequate attention must be paid to the dryingconditions.

Four types of polyimide films with thicknesses of 25 μm, 50 μm, 75 μm,and 100 μm (Sample D: elastic modulus 380 kg/mm², coefficient of waterabsorption>2.2%) were produced. Using these films, graphitization wasperformed also in Example 4 by the same method as that in Example 1.

The properties of the filmy graphites produced in Example 4 are shown inTable 5. As is evident from comparison between Tables 5 and 1, theproperties of the filmy graphites obtained in Example 4 are slightlyinferior to those in Example 1, but are superior to those in ComparativeExample.

TABLE 5 Starting film Heat treatment Coefficient of temperatureThickness linear expansion Electrical conductivity Thermal diffusivity(° C.) (μm) (×10⁻⁵/° C.) Birefringence (S · cm) (10⁻⁴ m²/s) 2700 25 2.00.13 10,500 8.0 50 2.0 0.13 10,000 7.8 75 2.0 0.13 9,200 7.8 100 2.10.13 8,000 7.8 2800 25 2.0 0.13 11,500 8.5 50 2.0 0.13 10,500 8.1 75 2.00.13 9,500 8.1 100 2.1 0.13 8,500 8.1

EXAMPLE 5

In Example 5, polyimide films manufactured by Kaneka Corporation andsold under the trade name of APICAL NPI with various thicknesses weregraphitized by the same method as that in Example 1.

APICAL NPI was produced as follows. Pyromellitic dianhydride (4equivalents) was dissolved in a solution prepared by dissolving 3equivalents of 4,4′-oxydianiline in DMF to synthesize a pre-polymerhaving acid anhydrides at both termini. Subsequently, by dissolving 1equivalent of p-phenylenediamine in a solution containing thepre-polymer, a solution containing 18.5% by weight of polyamic acid wasprepared.

While cooling the resulting solution, an imidization catalyst containing1 equivalent of isoquinoline relative to the carboxylic acid groupcontained in the polyamic acid and DMF was added thereto, followed bydefoaming. Subsequently, the resulting mixed solution was applied ontoan aluminum foil such that a predetermined thickness was achieved afterdrying. The mixed solution layer on a metal belt was dried using ahot-air oven and a far-infrared heater.

The drying conditions for achieving a final thickness of 75 μm were asfollows. The mixed solution layer on the metal belt was dried in ahot-air oven at 120° C. for 240 seconds to produce a self-supporting gelfilm. The resulting gel film was stripped off from the metal belt, andthe ends of the gel film were fixed. The gel film was dried by heatingstepwise in a hot-air oven at 120° C. for 30 seconds, at 275° C. for 40seconds, at 400° C. for 43 seconds, and at 450° C. for 50 seconds, andwith a far-infrared heater at 460° C. for 23 seconds. With respect toother thicknesses, the firing time was adjusted in proportion to thethickness. For example, in the case of a film with a thickness of 25 μm,the firing time was decreased to one third, compared with the case of 75μm.

Five types of sequential-controlled polyimide films with thicknesses of12.5 μm, 25 μm, 50 μm, 75 μm, and 125 μm (Sample E: elastic modulus 380kgf/mm², coefficient of water absorption 2.2%, birefringence 0.14,coefficient of linear expansion 1.6×10⁻⁵/° C.) were produced. Usingthese films, graphitization was performed also in Example 5 by the samemethod as that in Example 1 except that the heat treatment temperaturewas set at 2,800° C. or 3,000° C. in Example 5.

Various physical properties of the filmy graphites obtained in Example 5are shown in Table 6. As is evident from comparison between Table 6 andTables 1 to 5, the properties of the filmy graphites in Example 5 aresuperior not only to those of Comparative Example 1 but also to those ofExamples 1 to 4. Of course, as graphitization proceeds, the density andelectrical conductivity tend to increase. In Table 6, ρ(77K)/ρ(rt) andρ(4K)/ρ(rt) respectively indicate the ratio of electrical resistance at77 K to that at room temperature and the ratio of the electricalresistance at 4 K to that at room temperature. These electricalresistance ratios tend to decrease as graphitization proceeds.

Furthermore, the thermal conductivity and the thermal diffusivity tendto increase as graphitization proceeds, and these properties aredirectly important when the filmy graphite is used as a heat-dissipatingfilm. Here, the thermal conductivity (W/(m·K)) was calculated bymultiplying the thermal diffusivity (m²/s) times the density (kg/m³),and times the specific heat (theoretical value: 0.709 kJ/(kg·K)). Thedensity was calculated by dividing the weight by the volume.

The Raman spectrum intensity ratio in Table 6 is represented by theratio of the spectrum peak at a wave number of 1,310 cm⁻¹ correspondingto the diamond bond to the spectrum peak at a wave number of 1,580 cm⁻¹corresponding to the graphite bond. Of course, the lower spectrum peakratio indicates higher graphitization. In the Raman measurement, a lightbeam with a diameter of 10 μm was applied to the center of a crosssection in the thickness direction of the filmy graphite.

TABLE 6 Properties of filmy graphite Heat Thickness Raman treatment ofstarting Electrical Thermal Thermal spectrum intensity temperaturematerial Thickness Density conductivity ρ (77 K) ρ (4 K) conductivitydiffusivity ratio (° C.) (μm) (μm) (g/cm³) (S · cm) ρ (rt) ρ (rt) (W/m ·K) (×10⁻⁴ m²/s) (1310 cm⁻¹/1580 cm⁻¹) 2800 12.5 5.0 2.2 13500 — — 14819.5 — 25 10.3 2.19 13000 0.9 0.75 1398 9 — 50 21 2.17 13000 1.0 0.9 13059 — 75 33 2.14 12000 1.3 1.2 1500 9.9 0.38 125 55 2.10 12000  1.45 1.41486 10 0.41 3000 12.5 4.9 2.42 20000 — — 1767 10.3 — 25 10.6 2.41 200000.8 0.4 1760 10.3 — 50 20 2.38 16000 0.9  0.75 1725 10.2 — 75 32.5 2.1716000 1.1 1 1536 10 0.26 125 55.5 2.12 13000 1.2 1 1505 10 0.3 

FIG. 3 is a bright-field image of a filmy graphite in the vicinity of asurface layer, the filmy graphite being obtained by heat treatment at3,000° C. of the polyimide film with a thickness of 125 μm in Example 5,the image being observed with a transmission electron microscope (TEM).In the TEM observation, the filmy graphite was embedded in a protectiveresin to prepare a specimen for observing a cross section in thethickness direction of the graphite layer. In FIG. 3, arrows indicate aboundary between the protective resin and the graphite layer.

As is evident from the layered contrast of the TEM photograph, thegraphite layer has a single-crystal structure in which thecrystallographic (0001) plane (also referred to as the “c-plane”, ingeneral) is parallel to the surface. In FIG. 3, delaminations along thec-plane are observed. These delaminations are caused by unexpectedexternal force during the preparation and handling of the specimen forthe microscope. The fact that such delaminations easily occur means thatgraphite crystallization has proceeded to a high degree and breakingeasily occurs along the c-plane.

FIG. 4 is a crystal lattice image observed with a TEM in the vicinity ofthe surface layer corresponding to FIG. 3. In FIG. 4, a linear latticeimage corresponding to the c-plane of the graphite is shown with a clearcontrast and it can be confirmed that the linear lattice extendsparallel to the surface.

FIG. 5, similar to FIG. 3, is a bright-field image of a filmy graphitein the vicinity of a center in the thickness direction, the image beingobserved with a TEM. As is evident from the TEM photograph, the samegraphite single-crystal structure as that of the surface layer is alsoformed in the vicinity of the center in the thickness direction. In FIG.5, similar to FIG. 3, delaminations along the c-plane are observed. FIG.6 is a crystal lattice image observed with a TEM in the vicinity of thecenter in the thickness direction corresponding to FIG. 5. In FIG. 6,although linearity of the linear lattice image corresponding to thec-plane of the graphite is slightly inferior to that in FIG. 4,extending of the lines can be confirmed.

COMPARATIVE EXAMPLE 2

In Comparative Example 2, polyimide films manufactured by DuPont andsold under the trade name of Kapton H with various thicknesses weregraphitized by the same method as that in Example 5.

Although the production process of the Kapton film is not known, it isassumed that the Kapton film is produced by a method in which animidization catalyst composed of acetic anhydride, beta-picoline, andDMAc is added to a polyamic acid solution prepared by dissolving 1equivalent of 4,4′-oxydianiline in dimethylacetamide (DMAC) and furtherdissolving 1 equivalent of pyromellitic dianhydride therein.

The Kapton H has an elastic modulus of 330 kgf/mm², a coefficient ofwater absorption of 2.9%, a birefringence of 0.11, and a coefficient oflinear expansion of 2.7×10⁻⁵/° C.

Various physical properties of the filmy graphites obtained inComparative Example 2 are shown in Table 7. As is evident fromcomparison between Tables 7 and 6, the properties of the filmy graphitesin Example 5 are remarkably superior to the filmy graphites producedfrom the Kapton films in Comparative Example 2.

TABLE 7 Properties of filmy graphite Heat Thickness Raman treatment ofstarting Electrical Thermal Thermal spectrum intensity temperaturematerial Thickness Density conductivity ρ (77 K) ρ (4 K) conductivitydiffusivity ratio (° C.) (μm) (μm) (g/cm³) (S · cm) ρ (rt) ρ (rt) (W/m ·K) (×10⁻⁴ m²/s) (1310 cm⁻¹/1580 cm⁻¹) 2800 25 10.3 2.27 11000 1.1 1 8875.7 — 50 23.5 2.13 6900 1.2 1.1 680 4.5 — 75 41 1.89 3500 1.8 1.6 4423.3 0.41 125 100 1.42 980 2.5 2.8 238 2.3 0.44 3000 25 10.1 2.46 180001.05 0.8 1706 9.8 — 50 21 2.38 12000 1.2 1.1 1519 9.0 — 75 35 2.14 80001.7 1.5 1237 8.1 0.37 125 70 1.86 2800 2.1 2.5 660 5 0.4 

FIG. 7 is a bright-field image of a filmy graphite in the vicinity of asurface layer, the filmy graphite being obtained by heat treatment at3,000° C. of the Kapton (registered trademark) film with a thickness of125 μm in Comparative Example 2, the image being observed with a TEM. InFIG. 7, a single-crystal structure in which the c-plane of the graphiteis parallel to the surface occurs in the vicinity of the surface layer.FIG. 8 is a crystal lattice image observed with a TEM in the vicinity ofthe surface layer corresponding to FIG. 7. In FIG. 8, although linearityof the linear lattice image corresponding to the c-plane of the graphiteis slightly inferior to that in FIG. 4, extending of the lines can beconfirmed.

FIG. 9 is a bright-field image of a filmy graphite in the vicinity of acenter in the thickness direction, the image being observed with a TEM.In the TEM photograph, a layered structure is not formed in the vicinityof the center in the thickness direction unlike the surface layer, andthus it is evident that graphitization is insufficient. FIG. 10 is acrystal lattice image observed with a TEM in the vicinity of a center inthe thickness direction corresponding to FIG. 9. In FIG. 10, it can beconfirmed that the linear lattice image corresponding to the c-plane ofthe graphite is wavy and broken. This indicates that even ifgraphitization partially proceeds, the resulting graphite is in amicrocrystalline state or the c-plane orientation is not aligned. Thatis, with respect to the filmy graphite produced from the Kapton(registered trademark) film, even if heat treatment is performed at ahigh temperature of 3,000° C., graphitization does not sufficientlyproceed in the center in the thickness direction.

INDUSTRIAL APPLICABILITY

According to the present invention, in comparison with the conventionalpolymer graphitization process, it is possible to produce a thickerfilmy graphite, and graphitization is enabled at lower temperatures andin a shorter time when a polymer film with the same thickness isgraphitized.

1. A process for producing a filmy graphite comprising the steps of:forming a polyimide film having a thickness of 75 μm or less and abirefringence of 0.12 or more; and heat-treating the polyimide film at2,700° C. or higher, wherein the filmy graphite has an electricalconductivity of 9,200 S/cm or more.
 2. A process for producing a filmygraphite comprising the steps of: forming a polyimide film having athickness of 75 μm or less and a mean coefficient of linear expansion ofless than 2.5×10⁻⁵/° C. in a range of 100° C. to 200° C., the meancoefficient of linear expansion being in a planar direction of the film;and heat-treating the polyimide film at 2,400° C. or higher, wherein thefilmy graphite has an electrical conductivity of 12,000 S/cm or more. 3.A process for producing a filmy graphite comprising the steps of:forming a polyimide film having a thickness of 75 μm or less and a meancoefficient of linear expansion of less than 2.5×10⁻⁵/° C. in a range of100° C. to 200° C., the mean coefficient of linear expansion being in aplanar direction of the film, and having a birefringence of 0.12 ormore; and heat-treating the polyimide film at 2,400° C. or higher,wherein the filmy graphite has an electrical conductivity of 12,000 S/cmor more.
 4. A process for producing a filmy graphite comprising thesteps of: forming a polyimide film having a thickness of 100 μm or lessand a birefringence of 0.12 or more; and heat-treating the polyimidefilm at 2,700° C. or higher, wherein the filmy graphite has anelectrical conductivity of 8,000 S/cm or more.
 5. The process forproducing a filmy graphite according to claim 4, wherein the filmygraphite has an electrical conductivity of 9,300 S/cm or more.
 6. Theprocess for producing a filmy graphite according to claim 4, wherein thefilmy graphite has an electrical conductivity of 9,700 S/cm or more. 7.A process for producing a filmy graphite comprising the steps of:forming a polyimide film having a thickness of 50 μm or less and a meancoefficient of linear expansion of less than 2.5×10⁻⁵/° C. in a range of100° C. to 200° C. birefringence of 0.12 or more; and heat-treating thepolyimide film at 2,400° C. or higher, wherein the filmy graphite has anelectrical conductivity of 16,000 S/cm or more.
 8. A process forproducing a filmy graphite comprising the steps of: forming a polyimidefilm having a thickness of 200 μm or less and a birefringence of 0.12 ormore; and heat-treating the polyimide film at 2,700° C. or higher,wherein the filmy graphite has an electrical conductivity of 9,800 S/cmor more.